Rapid escalation of energy costs in recent years has spurred efforts to improve efficiency in plant operations. Distillation practices have received great attention, since the process of distillation, the predominant unit operation in the oil refining and chemical industries, consumes prodigious amounts of energy.
One energy-saving procedure is the practice of multiple stage, e.g., two-stage, condensation of the overhead vapors from a distillation column. Multi-stage condensation procedures allow energy savings by permitting the use of higher temperature at which heat may be removed from the overhead vapor. Such a process is described, for example, in U.S. Pat. No. 3,320,159. In at least one two-stage procedure for condensation of distillation column overhead vapors, the first stage of condensation is operated under conditions to condense just enough liquid to provide reflux for the distillation column, and the second stage of condensation is operated under conditions to provide the top product of the column. For two-stage condensation to be of value in heat recovery, the vapor or top product from the distillation column must be a multi-component mixture, and there must be a reasonable temperature spread between its dew point and bubble point, e.g., 20 degrees F. Most hydrocarbon distillations fit this requirement.
A very common commercial process for which two-stage condensation can be helpful is the separation of gasoline boiling range materials (often 400.degree. to 430.degree. F. ASTM end point) as the top product from a feed containing also kerosene, jet fuel, distillate fuel and gas oil components. Two-stage condensation procedures, however, have the disadvantages of added capital cost and increased complexity. In general, the process, as known, has required the use of two accumulators, with interconnecting piping, pumps, etc. U.S. Pat. No. 4,235,706, issued Nov. 25, 1980, and incorporated herein by reference, describes an improved process for distillation and multi-stage condensation of a multicomponent vapor, the process being characterized by the integration and utilization of a unique accumulation zone, thereby providing effective reflux and reduced capital costs.
In the typical distillation column, the operating pressure is set so that overhead vapor may be condensed with the available cooling medium, usually cooling water or air. Normally, it is advantageous to keep the pressure as low as possible to increase the relative volatility of the feed components and thus lessen the reflux required for a given separation. As a consequence, the top product from the column is relatively cool; typically, only 20.degree.-40.degree. F. above the temperature of the cooling medium. If this product must be further processed at a higher temperature, for example, by further distillation, it must be reheated. The present invention seeks to conserve energy present in the vapor fraction from the distillation, and thereby reduce the overall energy consumption of the procedure.